This invention pertains to methodology for preparing rubber-modified monovinylidene aromatic copolymers of the sort which are commonly referred to in the art as ABS resins. In particular, it pertains to a subcategory of such resins wherein the grafted rubber particles dispersed therein exhibit a particle size distribution which is multimodal in character (i.e., having 2 or more distinct peaks in the particle size distribution) and wherein at least two of the distinct particle size peaks are attributable to (i.e., composed of) emulsion graft polymerized rubber particles.
Rubber modified styrenic polymers such as acrylonitrile/butadiene/styrene (ABS) resins and high impact polystyrene (HIPS) resins are well known in the art and in industry and find use in a wide variety of practical applications including use in computer and business equipment housings, component parts of various household appliances, trim and other parts in the automobile industry, and the like.
Also known in the art at least as a general proposition is the fact that a range of different physical and aesthetic property combinations can be imparted to such rubber modified styrenic polymer compositions by adjusting or controlling the particle size and particle size distribution of the grafted elastomeric or rubbery particles that are dispersed within, and which impart toughness and impact strength to, such polymer compositions.
Thus, for example, in Aubrey's U.S. Pat. No. 3,509,235 compositions are described having bimodal particle size distributions wherein a first, relatively large sized (0.8-2 micron number average diameter) group of grafted rubber particles constitute a minor portion (from 3-30 weight percent) of the total grafted particles and is prepared by suspension graft polymerization techniques and wherein the major proportion by weight of the grafted rubber particles are prepared by emulsion graft polymerization and have a number average particle size of from 0.01 to 0.25 micron.
ABS resin compositions having bimodal grafted rubber particle size distributions have also been disclosed wherein the relatively smaller and the relatively larger-sized grafted rubber particles contained therein are both prepared by emulsion graft polymerization techniques. In such instances, the relatively large sized (e.g., having an average diameter of 0.25 micron or more) grafted rubber particles can be obtained by separately growing the underlying rubber latex to the desired large particle size using carefully controlled emulsion polymerization conditions (and, typically, for fairly prolonged reaction times) and thereafter blending said large-sized rubber latex with the small sized rubber latex either prior or subsequent to the desired emulsion graft polymerization thereof. See, for example, Ott et al.'s U.S. Pat. No. 4,009,227 and Kawashima et al.'s U.S. Pat. No. 5,008,331. Alternatively, a relatively smaller sized rubber latex (e.g., having an average diameter of 0.05 to 0.15.mu.) can be agglomerated to form the large size particles (or partially agglomerated to simultaneously provide both the large and small sized particles) prior to the emulsion graft polymerization process. (See, for example, Henton et al.'s U.S. Pat. No. 4,419,496.) Typically, when the relatively large sized emulsion graft polymerized rubber particles are separately"grown" to the desired size in their own independent emulsion polymerization step or operation, they are generally characterized as having a fairly narrow single or monomodal particle size distribution peak of their own which falls near the volume average particle size of the large particle size group taken as a whole. In contrast, when the large-sized particles are instead generated by agglomeration or partial agglomeration of smaller rubber latex particles, they typically exhibit a fairly broad or"polydisperse" particle size distribution which is spread out more or less evenly over the full size range of the individual agglomerated particles.
Known also in the art are so-called trimodal ABS compositions. Exemplary compositions of this type are described in Schmitt et al.'s U.S. Pat. No. 4,430,478 and Henton's U.S. Pat. No. 4,713,420 wherein there are disclosed trimodal ABS compositions which contain two different groups of emulsion graft polymerized rubber particles (one group having a relatively small size, e.g., averaging 0.25 micron or less and the other having an average size in excess of 0.25 micron) in combination with relatively large sized (e.g., in excess of 0.5 micron volume average) grafted rubber particles obtained by way of a mass, solution or suspension graft polymerization process.
While improvements in the overall balance of physical (e.g., impact strength, tensile strength, melt flow characteristics etc.) and aesthetic (e.g., gloss and surface appearance) properties can be obtained with the above-noted multimodal ABS compositions (i.e., relative to those of otherwise similar monomodal compositions), the general trend of further improvements in one property or characteristic such as impact strength coming only at the expense of some other property such as gloss and/or melt fluidity still prevails in the context of these various multimodal ABS resin compositions.
It would therefore be desirable to provide a means by which improvement in one or more properties within such compositions (e.g., melt fluidity and/or impact strength and/or gloss etc.) could be obtained without attendant significant sacrifices in the remaining properties of interest.